Methods and apparatus for maintaining synchronization of a polyphase motor during power interruptions

ABSTRACT

Methods and apparatus permit: monitoring a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses signals in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; converting kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and regulating a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to methods and apparatus formaintaining synchronization between a control circuit and a rotor of apolyphase motor during power interruptions, particularly when rotorposition sensors are not employed in the control and drive of thepolyphase motor.

[0002] Polyphase AC motors, such as permanent magnet, synchronousmachines must be driven such that the windings thereof are energized asa function of the rotor position (and, thus, the rotor flux) in order toobtain driving torque from the machine. Conventionally, the rotorposition is obtained by way of one or more rotor position sensors withinthe polyphase motor assembly, which sensors provide signals indicativeof the rotor position to a control circuit.

[0003] The material and labor costs associated with employing positionsensors within the polyphase motor assembly are undesirable and,therefore, techniques have been developed that permit properenergization of the windings of a polyphase motor without using positionsensors. Some of these techniques are discussed in, for example, U.S.Pat. Nos. 5,565,752; and 5,929,577, the entire disclosures of which arehereby incorporated by reference.

[0004] Control and drive techniques that do not require position sensorsshare a common characteristic, namely, that the rotor position of thepolyphase motor is unknown at startup. In order to deal with the unknownrotor position, these techniques employ an open-loop accelerationprocess where the windings of the polyphase motor are driven withoutsynchronization with the rotor position until the motor reaches athreshold rotational speed. At this speed, the polyphase motor generatessignals of sufficient magnitudes to provide an indication of the rotorposition. Among the signals that may be indicative of the rotor positionare the back electromotive force (BEMF) voltages of the windings, thewinding currents, etc.

[0005] Reference is now made to FIG. 1, which illustrates a blockdiagram of a conventional system 10 for controlling and driving apolyphase motor 18, which system measures the BEMF voltages of thepolyphase motor 18 to determine rotor position. The system 10 includes aDC source 12, a control circuit 14, a driver circuit 16, and thepolyphase motor 18. The DC source 12 produces a voltage, VDC, withrespect to ground, which is utilized to provide an operating DC voltage,VCC, to the control circuit 14 and to provide a DC bus voltage, VBUS, tothe driver circuit 16. The control circuit 14 provides commutationcontrol signals to the driver circuit 16 such that the driver circuit 16can properly energize the windings of the motor 18. The windings of themotor 18 (which are typically in the standard wye configuration, butwhich may also be in the delta configuration) are coupled to the drivercircuit 16 by way of nodes A, B, and C. The driver circuit 16 providesvarious current paths among these nodes, the DC bus, and ground in orderto drive the polyphase motor 18. The control circuit 14 monitors thevoltages at nodes A, B, and C, such as the BEMF voltages, and utilizessame to maintain synchronization with the rotor position of thepolyphase motor 18.

[0006] Unfortunately, the conventional techniques of monitoring signalsindicative of rotor position (such as the BEMF voltages) cannot maintainsynchronization with the polyphase motor 18 in the event of a powerinterruption, even if the power interruption is only momentary and themotor 18 has not stopped turning. This is so because during the powerinterruption the control circuit 14 is de-energized and looses allsynchronization information. This is best seen in FIG. 2, which is agraphical representation of the characteristics of the voltage at nodeA, the DC bus voltage, and the DC source voltage during a powerinterruption. At time t0, a power interruption occurs and the DC sourcevoltage, VDC, falls from about 24 volts to about 0 volts. Assuming thatthere is some impedance between the DC source 12 and the DC bus, the DCbus voltage, VBUS, (and VCC) falls after t0 as a function of the speedof the polyphase motor 18, which is decelerating. Likewise, the voltageat node A falls as a function of the slowing rotational speed of thepolyphase motor 18. When the operating DC voltage, VCC, has fallenbelow, for example, about 15 volts, the control circuit 14 ceases tofunction properly and loses synchronization with the rotor position ofthe polyphase motor 18.

[0007] When power is restored, resynchronization of the control circuit14 to the rotor position must be established in order to properlycommutate the windings of the polyphase motor 18. Among the conventionalprocesses for reestablishing synchronization is permitting the polyphasemotor 18 to stop rotating and restarting the polyphase motor 18utilizing the open-loop acceleration process discussed above. Thistechnique may be unsatisfactory for various reasons, including thedelays associated with stopping and restarting the polyphase motor 18,which are exacerbated when the inertias of the motor load and/or therotor itself are large.

[0008] Other techniques have been developed for reestablishingsynchronization between the control circuit and the rotor position,which techniques are set out in detail in U.S. Pat. Nos. 5,223,772;5,172,036; and 6,194,861, the entire disclosures of which are herebyincorporated by reference. These conventional techniques, however, allpresuppose that synchronization has been lost and must be reestablishedusing some specialized process. The manifest disadvantage of thesetechniques, therefore, is the reactive approach that they take to theloss of synchronization. Indeed, they do not address the root problem:the loss of synchronization itself.

[0009] Accordingly, there are needs in the art of new methods andapparatus for maintaining synchronization between a control circuit anda rotor of a polyphase motor during power interruptions, so long as themotor is rotating.

SUMMARY OF THE INVENTION

[0010] In accordance with one or more aspects of the present invention,a method includes: monitoring a level of a DC source that is used toprovide an operating DC voltage to a control circuit and to provide DCbus voltage to a driver circuit for a polyphase motor, the controlcircuit being of a type that senses signals in windings of the polyphasemotor to determine a rotor position thereof and to maintainsynchronization therewith; converting kinetic energy of the polyphasemotor into a secondary DC source when the level of the DC source hasfallen and reached a threshold; and regulating a voltage level of thesecondary DC source such that it is operable to provide the operating DCvoltage to the control circuit, and such that the control circuit iscapable of maintaining synchronization with the polyphase motor whilethe DC source remains substantially at or below the threshold.

[0011] By way of example, the motor may be a polyphase AC motor and thesignals indicative of rotor position may be the BEMF voltages of thewindings.

[0012] Preferably, the step of converting kinetic energy of thepolyphase motor comprises boosting the BEMF voltage to produce thesecondary DC source. To this end, the method may include: providingrespective paths for current to flow between pairs of the windings ofthe polyphase motor such that the current ramps up during some periodsof time (e.g., first periods of time); and interrupting the respectivepaths for current and providing other respective paths for the currentto flow between the pairs of the windings of the polyphase motor suchthat the current ramps down during other periods of time (e.g., secondperiods of time). For example, the current may be circulated to thesecondary DC source during at least one of the first and second periodsof time. Preferably, the current bypasses the secondary DC source duringthe first periods of time.

[0013] By way of example, a pulse width modulation regulator circuit maybe used to control the periods of time during which the respective pathsare provided and interrupted in response to the voltage level of thesecondary DC source. Alternatively, an aggregate ripple current of thecurrent flowing through the respective paths may be used to control theperiods of time during which the respective paths are provided andinterrupted.

[0014] In accordance with one or more further aspects of the presentinvention, an apparatus includes: a voltage sensing circuit operable tomonitor a level of a DC source that is used to provide an operating DCvoltage to a control circuit and to provide DC bus voltage to a drivercircuit for a polyphase motor, the control circuit being of a type thatsenses signals in windings of the polyphase motor to determine a rotorposition thereof and to maintain synchronization therewith; a boostcircuit operable to convert kinetic energy of the polyphase motor into asecondary DC source when the level of the DC source has fallen andreached a threshold; and a voltage regulator circuit operable to providesignaling to the boost circuit to regulate a voltage level of thesecondary DC source such that it is operable to provide the operating DCvoltage to the control circuit, and such that the control circuit iscapable of maintaining synchronization with the polyphase motor whilethe DC source remains substantially at or below the threshold.

[0015] Preferably, the boost circuit is operable to boost the BEMFvoltage on the windings of the polyphase motor to produce the secondaryDC source. To this end, the boost circuit may include a plurality ofcommutation elements that are controlled to: provide respective pathsfor current to flow between pairs of the windings of the polyphase motorsuch that the current ramps up during some periods of time (e.g., firstperiods of time); and interrupt the respective paths for current andproviding other respective paths for the current to flow between thepairs of the windings of the polyphase motor such that the current rampsdown during other periods of time (e.g., second periods of time). Again,the current may be circulated to the secondary DC source during at leastone of the first and second periods of time. Preferably, the currentbypasses the secondary DC source during the first periods of time.

[0016] By way of example, the driver circuit may include respectivepairs of high-side and low-side switches coupled in series across the DCbus and coupled at respective intermediate nodes to the windings of thepolyphase motor, each switch including an anti-parallel diodethereacross. In such a case, the boost circuit is preferably operable touse at least some of the anti-parallel diodes to provide the paths forcurrent to flow between the pairs of the windings of the polyphasemotor.

[0017] Preferably, the commutation elements include respectivecommutating switches, coupled from the intermediate nodes to a commonnode of the low-side switches, to provide the paths for current to flowbetween the pairs of the windings of the polyphase motor, and for thecurrent to ramp up during some periods of time. For example, thecommutating switches may include respective diodes, each having an anodecoupled to one of the intermediate nodes and having a cathode coupled tothe common node of the low-side switches through a switch.Alternatively, the commutating switches may include respectivetransistors coupled from the intermediate nodes to the common node ofthe low-side switches. Still further, the commutating switches may be:(i) two or more of the low-side switches; (ii) two or more of thehigh-side switches; or (iii) one of the high-side switches and one ofthe low-side switches (in a manner where the DC bus voltage aids theBEMF voltage), which are operable to turn on to provide the paths forcurrent to flow between the pairs of the windings of the polyphasemotor, and for the current to ramp up during the first periods of time.

[0018] Preferably, the current is circulated to the secondary DC sourceat least one of the first and second periods of time. It is mostpreferred that the current bypasses the secondary DC source during thefirst periods of time.

[0019] Preferably, during a motoring mode, the control circuit isoperable to provide commutation control signals to the driver circuitsuch that the windings are commutated with respect to the DC bus voltageto cause the polyphase motor to produce motoring torque; and at leastone of the voltage sensing circuit and the voltage regulator circuit isoperable to inhibit the control circuit from providing the motoringcommutation control signals to the driver circuit while the DC sourceremains substantially at or below the threshold. Further, it ispreferred that the at least one of the voltage sensing circuit and thevoltage regulator circuit is operable to enable the control circuit toprovide the motoring commutation control signals to the driver circuitwhen the DC source rises substantially to or above the threshold,wherein the enabling may be carried out without first stopping andrestarting the polyphase motor.

[0020] Other advantages, features, and aspects of the invention will beapparent to one skilled in the art in view of the discussion hereintaken in conjunction with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] For the purposes of illustrating the invention, there are shownin the drawings forms that are presently preferred, it being understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown.

[0022]FIG. 1 is a conceptual block diagram illustrating a conventionaltechnique for controlling and driving a polyphase motor;

[0023]FIG. 2 is a graphical representation of certain voltages in theblock diagram of FIG. 1 under power interrupt conditions;

[0024]FIG. 3 is a block diagram illustrating a system for controllingand driving a polyphase motor in accordance with one or more aspects ofthe present invention;

[0025]FIG. 4 is a graphical representation of certain voltages in thesystem of FIG. 3 under power interrupt conditions;

[0026]FIG. 5 is a block diagram illustrating an alternative system forcontrolling and driving a polyphase motor in accordance with one or morefurther aspects of the present invention;

[0027]FIG. 6 is an example of a more detailed circuit implementation ofcertain portions of the system of FIG. 5;

[0028]FIG. 7 is a block diagram of a further alternative system forcontrolling and driving a polyphase motor in accordance with one or morefurther aspects of the present invention;

[0029]FIG. 8 is an example of a more detailed implementation of certainportions of the system of FIG. 7; and

[0030]FIG. 9 is a graphical representation of certain signals andconditions of the system of FIG. 7 under power interrupt and powerreacquisition conditions.

DETAILED DESCRIPTION

[0031] Referring now to the drawings, wherein like numerals indicatelike elements, there is shown in FIG. 3 a block diagram illustrating oneor more aspects of the present invention. For the purposes of brevityand clarity, the block diagram of FIG. 3 will be referred to, anddescribed herein, as illustrating a system 100, it being understood,however, that the description may be readily applied to various aspectsof one or more methods of the present invention with equal force. Thesystem 100 preferably includes a DC source 12, a switch 102, a controlcircuit 104, a driver circuit 16, and a power interrupt (orride-through) circuit 110, which all cooperate to commutate the windingsof a polyphase motor 18.

[0032] It is noted that the polyphase motor 18 may be a permanent magnet(PM) machine, such as a polyphase AC motor, a brushless DC motor, etc.,or an induction machine. The illustrative embodiments described hereinwere subject to experimentation and/or testing in connection with abrushless DC polyphase motor 18. It is understood, however, that skilledartisans can easily apply the details of these illustrative embodimentsin connection with other types of machines.

[0033] The DC source produces a voltage, VDC, that is input to theswitch 102. The switch 102 is preferably operable to disconnect the DCsource 12 from the control circuit 104 and the driver circuit 16 undercertain conditions, such as during a power interruption. The switch 102may be implemented utilizing any of the known techniques, such as by wayof one or more diodes, one or more transistors, one or more relays, etc.In normal operation, however, the switch 102 permits the DC source 12 toprovide an operating DC voltage, VCC, to the control circuit 104 and toprovide a DC bus voltage, VBUS, to the driver circuit 16.

[0034] The invention contemplates that the normal voltage level of theDC source 12 may take on any value. When the voltage level of the DCsource 12 is relatively low, such as 24 volts, the DC source 12 maydirectly provide the operating DC voltage to the control circuit 104, asis shown in FIG. 3. When the voltage level of the DC source 12 is higherthan the maximum voltage rating for the operating DC voltage level ofthe control circuit 104, however, an additional voltage regulatingdevice (not shown) may be necessary between the DC source 12 and thecontrol circuit 104 to provide the operating DC voltage.

[0035] During a motoring mode of operation, the control circuit 104 isoperable to provide commutation control signals to the driver circuit 16such that the windings of the polyphase motor 18 are commutated in a waythat causes the motor 18 to produce motoring torque. The control circuit104 monitors signals of the windings (i.e., at nodes A, B, and C) inorder to establish synchronization with the rotor position of thepolyphase motor 18 and to issue proper commutation control signals tothe driver circuit 16. Any of the known techniques for monitoring suchsignals may be employed, for example, monitoring BEMF voltages,monitoring current levels in the windings, etc.

[0036] The power interrupt circuit 110 is coupled to various nodes ofthe system 100 in order to permit the control circuit 104 to maintainsynchronization with the rotor position of the polyphase motor 18 duringa power interrupt condition. Although the invention is not limited byany theory of operation, it is preferred that a power interruptcondition exists when the voltage level of the DC source 12, VDC,reaches or falls below a threshold. It is noted, however, that otherindicators of a power interrupt may exist, such as a low voltage levelon VCC, VBUS, or some other node. More particularly, the power interruptcircuit 110 permits the conversion of kinetic energy of the polyphasemotor 18 (i.e., the energy associated with the rotational inertia of therotor and load) into a secondary DC source during a power interrupt. Thepower interrupt circuit 110 also permits the system 100 to regulate thevoltage level of this secondary DC source such that it is operable toprovide the operating DC voltage, VCC, to the control circuit 104 in away that the control circuit 104 is capable of maintainingsynchronization with the rotor of the polyphase motor 18.

[0037] To this end, the power interrupt circuit 110 preferably includesa voltage sensing circuit 112, a voltage regulation and control circuit114, and a commutation circuit 116. The voltage sensing circuit 112 ispreferably operable to monitor a voltage level of the DC source 12 byway of signaling on line 112A and determining whether that level hasreached (or fallen below) a threshold. Reference is now made to FIG. 4,which is a graphical representation of various signals of the system100. (It is noted that FIG. 4 represents actual test data of anillustrative embodiment of the invention.) The voltage sensing circuit112 is preferably operable to detect that the voltage level of the DCsource 12, VDC, has fallen below a threshold at time t0. The voltagesensing circuit 112 preferably provides signaling to at least one of thevoltage regulation and control circuit 114 and the control circuit 104,indicating that a loss of power condition exists.

[0038] As it is undesirable for the control circuit 104 to providecommutation signaling to the driver circuit 16 during the powerinterrupt, at least one of the voltage sensing circuit 112 and thevoltage regulation and control circuit 114 preferably provide adisabling signal via line 114A to the control circuit 104 during thepower interrupt. By way of example, the control circuit 104 may includesuitable digital logic circuitry (or analog circuitry), eitherinternally or externally, which interrupts the commutation signaling tothe driver circuit 16 in response to the disabling signal on line 114A.Any of the known circuit techniques may be employed to implement suchdigital logic and/or analog circuitry.

[0039] The voltage regulation and control circuit 114 and thecommutation circuit 116 preferably performs a voltage boost function anda voltage regulator function in order to convert the kinetic energy ofthe polyphase motor 18 into a secondary DC source capable of providingthe operating DC voltage, VCC, to the control circuit 104 during thepower interrupt. More particularly, the boost function boosts the BEMFvoltage of the windings of the polyphase motor 18 in a manner dictatedby the voltage regulation and control circuit 114 to produce thesecondary DC source. The voltage regulation and control circuit 114ensures that the voltage level of the secondary DC source, e.g., onVBUS, is well regulated by sensing VBUS via line 114B. This is shown inFIG. 4, whereby the voltage level of VBUS (and, therefore, VCC) isregulated to, for example, 15 volts during the power interrupt. (This isin sharp contrast to permitting the bus voltage, VBUS, to ramp down as afunction of the speed of the polyphase motor 18 as in the prior art, seeFIG. 2.) Thus, the control circuit 104 is capable of maintainingsynchronization with the rotor position of the polyphase motor 18 eventhough power has been interrupted.

[0040] To achieve the boost function, the commutation circuit 116 ispreferably operable to circulate the currents flowing into and out ofthe polyphase motor 18 such that a net accumulation of charge may beobtained and stored for use in producing the secondary DC source. Moreparticularly, the commutation circuit 116 is preferably operable to: (i)provide respective paths for current to flow between pairs of thewindings of the polyphase motor 18 such that the current ramps up duringsome periods of time; and (ii) interrupt these respective paths forcurrent and provide other respective paths for the current to flowbetween the pairs of the windings of the polyphase motor 18 such thatthe current ramps down during other periods of time.

[0041] Further details of providing and interrupting these current pathswill be discussed in greater detail hereinbelow. At this point, however,it is noted that the provision and interruption of these current pathsare preferably carried out by way of a plurality of commutationswitching elements. These commutation switching elements may be entirelycontained within the commutation circuit 116, i.e., they may be separatefrom the driver circuit 16. Alternatively, the commutation switchingelements may be partially contained within the commutation circuit 116,i.e., at least some elements of the driver circuit 16 may be utilized asthe commutation switching elements. Still further, the commutationswitching elements might not be contained in the commutation circuit 116at all, i.e., all the commutation switching elements may be containedwithin the driver circuit 16. In at least the latter case, thecommutation circuit 116 preferably provides control signaling to thecommutation switching elements within the driver circuit 16 by way ofline 116A.

[0042] Further details concerning one embodiment of the commutationcircuit 116 will now be described with reference to FIG. 5, which is ablock diagram illustrating an example of a system 150 suitable forcarrying out one or more aspects of the present invention. In thisexample, the switch 102 is preferably implemented utilizing a diode,which prohibits current to flow back into the DC source 12 during apower interrupt condition. The voltage sensing circuit 112 monitors thevoltage level of the DC source 12 via line 112A and provides signalingindicating a loss of power condition to the voltage regulation andcontrol circuit 114 by way of line 112B. The voltage regulation andcontrol circuit 114 monitors the voltage level of the secondary DCsource, which in this example is the voltage level of the operating DCvoltage VCC and the DC bus voltage VBUS. It is noted that this is thesame voltage across a bulk capacitance, C, which is typically present toprovide local energy storage for the driver circuit 16 (and/or othercircuits).

[0043] The driver circuit 16 includes respective pairs of high-side andlow-side switches 16A-16B, 16C-16D, and 16E-16F. Each switch 16A-Fincludes an anti-parallel diode thereacross. The respective pairs ofswitches 16A-16B, 16C-16D, and 16E-16F are coupled in series across theDC bus VBUS and coupled at respective intermediate nodes A, B, and C tothe windings of the polyphase motor 18.

[0044] The system 150 includes a torque control circuit 120 operable toproduce signaling on line 120A, which the control circuit 104 uses tocause the polyphase motor 18 to produce a level of torque that is afunction of a torque command, Tin. More particularly, the torque controlcircuit 120 employs a hysteretic current mode technique, whereby anaggregate offset current level and a ripple current of the windings ofthe polyphase motor 18 are sensed by way of a current sensing circuit126. The offset current level is used to ensure that the polyphase motor18 is producing the commanded torque. The ripple current is used toprovide timing information to the control circuit 104 such that itproduces the requisite commutation signaling to the switches 16A-F ofthe driver circuit 16. These and other operational details of the torquecontrol circuit 120 may be found in U.S. Pat. No. 6,342,769, the entiredisclosure of which is hereby incorporated by reference.

[0045] In this illustrative embodiment, the commutation circuit 116includes some of the commutation switching elements for providing thecurrent paths for boosting the BEMF voltages of the polyphase motor 18.The driver circuit 16 includes other commutation switching elements toprovide such current paths. More particularly, the commutation circuit116 preferably includes respective diodes 124, each having an anodecoupled to one of the intermediate nodes A, B, and C, and having acathode coupled to the common node (e.g., ground) of the low-sideswitches 16B, 16D, and 16F through a switch 122. The diodes 124, theswitch 122, and the anti-parallel diodes of the driver circuit 16, whenproperly controlled, provide the current paths for boosting the BEMFvoltages of the polyphase motor 18 to produce the secondary DC source.

[0046] A detailed discussion of the operation of the system 150 underpower interruption conditions will now be provided. The voltage sensingcircuit 112 determines that a power interruption has occurred, andplaces the system 150 into a regenerative mode. To this end, the voltageregulation and control circuit 114 provides a signal on line 114A to thetorque control circuit 120 that reverses the polarity of the commandedtorque. In general, to maintain synchronization, the commutation of thewindings of the polyphase motor 18 must be such that the boosted BEMFvoltages are sufficiently high to both provide the operating DC voltage,VCC, to the control circuit 104, and to be sensed by the control circuit104.

[0047] An example of how the windings of the polyphase motor 18 may becommutated to convert the kinetic energy of the motor 18 (and/or load)into the secondary DC source will now be discussed. This example isgiven by way of illustration and not by way of limitation. Indeed, asdiscussed later in this specification, other examples exist and arecontemplated by the invention. When in the regenerative mode, currentsare ramped-up in the polyphase motor 18 by shorting the windingstogether for a period of time and then removing the shorting conditionand permitting the current to circulate through the bulk storagecapacitor C in a controlled manner. For example, when the torque controlcircuit 120 senses that the ripple current (as monitored by the currentsensing circuit 126) has fallen to a sufficiently low level, the switch122 will be turned on by way of signaling over line 120B. This placesthe cathodes of the diodes 124 substantially at ground potential andpermits current flow from one of the windings of the polyphase motor 18having a highest voltage potential to one of the windings having alowest voltage potential.

[0048] By way of example, it is assumed that the winding having thehighest potential is coupled to node C and the winding having the lowestpotential is coupled to node B. Thus, current will flow from node Cthrough the associated diode 124, through the switch 122, through thecurrent sensing circuit 126, through the anti-parallel diode of switch16D, to node B. It is noted that this current path provides for currentflow between the pairs of windings associated with nodes B and C, butbypasses the secondary DC source. In other words, the flowing currentdoes not place any charge on the bulk storage capacitor C, nor does itsource current into the control circuit 104 to provide operating DCvoltage thereto. The current in this path, however, ramps-up andeventually reaches a level at which the torque control circuit 120 turnsoff the switch 122 by way of line 120B.

[0049] When the switch 122 is turned off, the existing current path isinterrupted and the voltages induced in the windings of the polyphasemotor 18 reverse. The current flow in these windings, however, continuesto flow by way of another current path. In accordance with at least oneaspect of the present invention, the path between the windingsassociated with nodes B and C during this time interval permits currentto flow to the secondary DC source. In particular, current flows fromnode C, through the anti-parallel diode associated with switch 16Ethrough the bulk storage capacitor C into ground, through the currentsensing circuit 126, through the anti-parallel diode associated withswitch 16D, and to node B. During this time interval, voltages inducedin the windings of the polyphase motor 18 are additive with the BEMFvoltages, thereby providing a boost, and charge is delivered to thesecondary DC source, e.g., the charge on the bulk capacitor C increases.

[0050] While two current paths have been discussed in the above example,one skilled in the art will appreciate from the description herein thatother combinations of current paths will exist at subsequent timeintervals depending on the polarities and magnitudes of the voltages ofthe windings of the polyphase motor 18. All the while, the voltageregulation and control circuit 114 will adjust the commandedregeneration torque by way of line 114A such that the voltage level ofthe secondary DC source (e.g., the voltage across the bulk capacitanceC) is maintained at a desired level, such as 15 volts (FIG. 4).

[0051] With reference to FIG. 6, more detailed circuit diagrams areshown that are suitable for implementing the voltage sensing circuit112, the voltage regulation and control circuit 114, and the torquecontrol circuit 120 of FIG. 5. As the operation of these circuits willbe readily apparent to one skilled in the art from the descriptionherein, for the purposes of brevity a detailed discussion of same isomitted.

[0052] Reference is now made to FIG. 7, which illustrates an alternativesystem 200 that is suitable for carrying out one or more further aspectsof the present invention. In the system 200, the voltage regulationfunction is carried out by a pulse width modulation (PWM) voltageregulation circuit 214. While a hysteretic torque control circuitsimilar to that of FIG. 5 may be employed in the system 200, it is notutilized in the regulation of the secondary DC source during a powerinterrupt. Rather, the separate PWM voltage regulation circuit 214operates to control the switch 122 in order to regulate the voltage ofthe secondary DC source. The advantage of this approach is that the PWMvoltage regulation circuit 214 may be programmed to operate at asubstantially higher frequency than, for example, the hysteretic torquecontrol circuit 120 (FIG. 5) and, therefore, the current ripple in thesystem 200 may be reduced. In the system 200 of FIG. 7, the voltagesensing circuit 212 provides a disable signal to the control circuit 104by way of line 114A during the power interrupt condition. In other ways,the operation of the system 200 is similar to the system 150 discussedhereinabove with respect to FIG. 5.

[0053] With reference to FIG. 8, an example of a more detailed circuitimplementation of the PWM voltage regulation circuit 214 is provided. Itis understood, however, that the regulation circuit 214 may beimplemented in many different ways without departing from the spirit andscope of the invention as claimed. As the detailed operation of thecircuit illustrated in FIG. 8 will be apparent to one skilled in theart, for the purposes of brevity a detailed description thereof isomitted. It is noted, however, that an alternative implementation of thecommutation switching elements of the commutation circuit 116 isemployed as compared with the system 150 of FIG. 5. In particular, thethree diodes 124 and the switch 122 have been replaced with threeN-channel field effect transistors (FETs), which are controlled toperform the same function.

[0054] It is noted that the specific circuit implementation of thecommutation circuit 116 may take on many forms and, indeed, are toonumerous to reproduce in this description without sacrificingpracticality, brevity, and clarity. By way of further example, however,it is noted that the low-side switches 16B, 16D, and 16F of the drivercircuit 16 may be turned on in order to short the windings of thepolyphase motor 18 together and provide paths for the currents toramp-up in the polyphase motor 18. In this way, neither the diodes 124nor the switch 122 need be provided; however, appropriate controlsignaling must issue from the commutation circuit 116 (or any otherappropriate circuit) to turn on and to turn off such switches at theappropriate times. This is illustrated by way of line 116A in FIG. 3. Itis noted, however, that without the additional switching components ofthe commutation circuit 116, e.g., the diodes 124 and the switch 122,the current would not flow through the current sensing circuit 126.Thus, an open-loop voltage regulation technique and/or pure voltage modecontrol might be required to regulate the secondary DC source.Alternatively, separate current sensors could be utilized to monitor thecurrent flowing in each of the low-side switches 16B, 16D, 16F (and theassociated anti-parallel diodes) in order to sense the current flow andenable the use of current mode regulation techniques.

[0055] In yet another example, the commutation circuit 116 may beimplemented such that any two or more of the high-side switches 16A, 16Cand 16E are turned on to ramp-up the current in the polyphase motor 18.Again, in this example, neither the diodes 124 nor the switch 122 needbe provided; however, appropriate control signaling must issue from thecommutation circuit 116 (or any other appropriate circuit) to turn onand to turn off such switches at the appropriate times. This techniqueis also characterized in that the current would not flow through thecurrent sensing circuit 126 and, therefore, as discussed in the previousexample, an open-loop voltage regulation technique and/or pure voltagemode control might be necessary to regulate the secondary DC source.Alternatively, separate current sensors could also be utilized tomonitor the current flowing in each of the high-side switches 16A, 16C,16E (and the associated anti-parallel diodes) in order to sense thecurrent flow and enable the use of current mode regulation techniques.

[0056] In still another example, the commutation circuit 116 may beimplemented such that one of the high-side switches 16A, 16C, 16E andone of the low-side switches 16B, 16D, 16F are turned on in a mannerwhere the DC bus voltage aids the BEMF voltage to ramp-up the current inthe polyphase motor 18 during some periods of time. Thus, some charge isdrawn from the bulk capacitor C during these time periods. During otherperiods of time, however, all switches 16A-16F are turned off such thatthe current circulates through respective pairs of the anti-paralleldiodes and places charge on the bulk capacitor C. This technique wouldlikely be suitable when the polyphase motor 18 is an induction machine.

[0057] With reference to FIG. 9, and in accordance with one or morefurther aspects of the present invention, the systems 100, 150, and 200discussed hereinabove with respect to FIGS. 3, 5, and 7 are preferablyoperable to permit the control circuit 104 to maintain synchronizationwith the rotor position of the polyphase motor 18 during the powerinterrupt and substantially instantaneously accelerate the polyphasemotor 18 when power is reacquired. More particularly, FIG. 9 graphicallyillustrates the voltage level of the DC source, VDC, the voltage of nodeA, the speed of the polyphase motor 18, and the voltage level of thesecondary DC source (e.g., VBUS). At time t0 the voltage level of the DCsource 12 falls to zero and the speed of the polyphase motor 18 rampsdown. The power interrupt circuit 110 operates to convert the kineticenergy of the rotating polyphase motor 18 into the secondary DC sourceof voltage, and to regulate such voltage to a level sufficient to permitthe control circuit 104 to maintain synchronization with the rotor ofthe polyphase motor 18. At time t1, the voltage level of the DC source12 is reacquired and the power interrupt circuit 110 enables the controlcircuit 104 to resume normal operation. Advantageously, however, thecontrol circuit 104 need not reacquire synchronization with the rotor ofthe polyphase motor 18 because the control circuit 104 never lostsynchronization during the power interrupt. Consequently, the polyphasemotor 18 may be substantially immediately re-accelerated to the desiredspeed and/or torque.

[0058] It is noted that the methods and apparatus for maintainingsynchronization between the control circuit 104 and the rotor of thepolyphase motor 18 described hereinabove may be achieved utilizingsuitable hardware, such as that shown in the drawings. It is noted thatsuch hardware may be implemented utilizing any of the knowntechnologies, such as standard digital circuits, analog circuits, any ofthe known processors that are operable to execute software and/orfirmware programs, one or more programmable digital devices or systems,such as programmable read only memories (PROMs), programmable arraylogic devices (PALs), any combination of the above, etc. Indeed, whilevarious circuit implementations of the embodiments of the presentinvention may have advantages and disadvantages, they are all within thespirit and scope of the invention as claimed.

[0059] Further, although the invention herein has been described withreference to particular embodiments, it is to be understood that theseembodiments are merely illustrative of the principles and applicationsof the present invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method, comprising: monitoring a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses signals in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; converting kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and regulating a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.
 2. A method, comprising: monitoring a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses back electromotive force (BEMF) in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; converting kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and regulating a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.
 3. The method of claim 2, wherein the step of converting kinetic energy of the polyphase motor comprises boosting the BEMF voltage to produce the secondary DC source.
 4. The method of claim 3, further comprising: providing respective paths for current to flow between pairs of the windings of the polyphase motor such that the current ramps up during respective first periods of time; and interrupting the respective paths for current and providing other respective paths for the current to flow between the pairs of the windings of the polyphase motor such that the current ramps down during respective second periods of time.
 5. The method of claim 4, wherein the current is circulated to the secondary DC source during at least one of the first and second periods of time.
 6. The method of claim 4, wherein the current bypasses the secondary DC source during the first periods of time.
 7. The method of claim 4, further comprising using a pulse width modulation regulator circuit to control the periods of time during which the respective paths are provided and interrupted in response to the voltage level of the secondary DC source.
 8. The method of claim 4, further comprising using an aggregate ripple current of the current flowing through the respective paths to control the periods of time during which the respective paths are provided and interrupted.
 9. The method of claim 4, wherein: the driver circuit includes respective pairs of high-side and low-side switches coupled in series across the DC bus and coupled at respective intermediate nodes to the windings of the polyphase motor, each switch including an anti-parallel diode thereacross; and at least some of the anti-parallel diodes are used to provide the paths for current to flow between the pairs of the windings of the polyphase motor.
 10. The method of claim 9, further comprising using a commutation circuit to provide the paths for current to flow between the pairs of the windings of the polyphase motor such that the current ramps up during the first periods of time.
 11. The method of claim 10, wherein the commutation circuit includes respective commutating switches coupled from the intermediate nodes to a common node of the low-side switches.
 12. The method of claim 9, further comprising turning on (i) two or more of the low-side switches; (ii) two or more of the high-side switches; or (iii) one of the high-side switches and one of the low-side switches such that the DC bus voltage aids the BEMF, to provide the paths for current to flow between the pairs of the windings of the polyphase motor such that the current ramps up during the first periods of time.
 13. The method of claim 2, wherein, during a motoring mode, the control circuit is operable to provide motoring commutation control signals to the driver circuit such that the windings are commutated with respect to the DC bus voltage to cause the polyphase motor to produce motoring torque, the method further comprising: inhibiting the control circuit from providing the motoring commutation control signals to the driver circuit while the DC source remains substantially at or below the threshold.
 14. The method of claim 13, further comprising enabling the control circuit to provide the motoring commutation control signals to the driver circuit when the DC source rises substantially to or above the threshold, wherein the step of enabling may be carried out without first stopping and restarting the polyphase motor.
 15. An apparatus, comprising: a voltage sensing circuit operable to monitor a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses signals in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; a boost circuit operable to convert kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and a voltage regulator circuit operable to provide signaling to the boost circuit to regulate a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.
 16. An apparatus, comprising: a voltage sensing circuit operable to monitor a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses back electromotive force (BEMF) in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; a boost circuit operable to convert kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and a voltage regulator circuit operable to provide signaling to the boost circuit to regulate a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.
 17. The apparatus of claim 16, wherein the boost circuit is operable to boost the BEMF voltage on the windings of the polyphase motor to produce the secondary DC source.
 18. The apparatus of claim 17, wherein the boost circuit includes a plurality of commutation elements that are controlled to: provide respective paths for current to flow between pairs of the windings of the polyphase motor such that the current ramps up during respective first periods of time; and interrupt the respective paths for current and providing other respective paths for the current to flow between the pairs of the windings of the polyphase motor such that the current ramps down during respective second periods of time.
 19. The apparatus of claim 18, wherein the commutation elements are controlled such that the current is circulated to the secondary DC source during at least one of the first and second periods of time.
 20. The apparatus of claim 18, wherein the commutation elements are controlled such that the current bypasses the secondary DC source during the first periods of time.
 21. The apparatus of claim 18, wherein the voltage regulator circuit includes a pulse width modulation regulator operable to provide the signaling to the commutation elements, in response to the voltage level of the secondary DC source, to control the periods of time during which the respective paths are provided and interrupted.
 22. The apparatus of claim 18, wherein the voltage regulator circuit includes a hysteretic current regulator operable to provide the signaling to the commutation elements, in response to the voltage level of the secondary DC source and to an aggregate ripple current of the current flowing through the respective paths, to control the periods of time during which the respective paths are provided and interrupted.
 23. The apparatus of claim 18, wherein: the driver circuit includes respective pairs of high-side and low-side switches coupled in series across the DC bus and coupled at respective intermediate nodes to the windings of the polyphase motor, each switch including an anti-parallel diode thereacross; and the boost circuit is operable to use at least some of the anti-parallel diodes to provide the paths for current to flow between the pairs of the windings of the polyphase motor.
 24. The apparatus of claim 23, wherein the commutation elements include respective commutating switches, coupled from the intermediate nodes to a common node of the low-side switches, to provide the paths for current to flow between the pairs of the windings of the polyphase motor such that the current ramps up during the first periods of time.
 25. The apparatus of claim 24, wherein the commutating switches include respective diodes, each having an anode coupled to one of the intermediate nodes and having a cathode coupled to the common node of the low-side switches through a switch.
 26. The apparatus of claim 24, wherein the commutating switches include respective transistors coupled from the intermediate nodes to the common node of the low-side switches.
 27. The apparatus of claim 23, wherein the commutation switches are (i) two or more of the low-side switches; (ii) two or more of the high-side switches; or (iii) one of the low-side switches and one of the high-side switches such that the DC bus voltage aids the BEMF, which are operable to turn on to provide the paths for current to flow between the pairs of the windings of the polyphase motor such that the current ramps up during the first periods of time.
 28. The apparatus of claim 16, wherein: during a motoring mode, the control circuit is operable to provide motoring commutation control signals to the driver circuit such that the windings are commutated with respect to the DC bus voltage to cause the polyphase motor to produce motoring torque; and at least one of the voltage sensing circuit and the voltage regulator circuit is operable to inhibit the control circuit from providing the motoring commutation control signals to the driver circuit while the DC source remains substantially at or below the threshold.
 29. The apparatus of claim 28, wherein the at least one of the voltage sensing circuit and the voltage regulator circuit is operable to enable the control circuit to provide the motoring commutation control signals to the driver circuit when the DC source rises substantially to or above the threshold, wherein the enabling may be carried out without first stopping and restarting the polyphase motor. 